Enhanced osmotic transport in individual double-walled carbon nanotube

The transport of fluid and ions across nanotubes or nanochannels has attracted great attention due to the ultrahigh energy power density and slip length, with applications in water purification, desalination, energy conversion and even ion-based neuromorphic computing. Investigation on individual nanotube or nanochannel is essential in revealing the fundamental mechanism as well as demonstrating the property unambiguously. Surprisingly, while carbon nanotube is the pioneering and one of the most attractive systems for nanofluidics, study on its response and performance under osmotic forcing is lacking. Here, we measure the osmotic energy conversion for individual double-walled carbon nanotube with an inner radius of 2.3 nm. By fabricating a nanofluidic device using photolithography, we find a giant power density (up to 22.5 kW/m2) for the transport of KCl, NaCl, and LiCl solutions across the tube. Further experiments show that such an extraordinary performance originates from the ultrahigh slip lengths (up to a few micrometers). Our results suggest that carbon nanotube is a good candidate for not only ultrafast transport, but also osmotic power harvesting under salinity gradients.


Fabrication process of individual carbon nanotube nanofluidic chip
. Fabrication process of individual carbon nanotube nanofluidic chip.
The specific markers that indicate the position of the CNT on silicon wafer are etched by RIE technique. Ultra-long horizontally aligned CNTs are grown using CVD. Only one CNT is remained as the specific nanochannel in the device and protected using positive photoresist, whereas all other CNTs are removed by oxygen plasma etching process. SU-8 photoresist is used to fabricate the two independent microchannels for reservoirs of ionic solution and a mask against plasma etching. By alignment lithography technology, the microfluidic channels are precisely constructed on silicon wafer. The FIB precise etching technology removes the exposed parts of the CNT and opens both ends of CNT underneath the epoxy wall, which connects the two microchannels. The PDMS is bonded on the SU8 microchannels after being immersed in an aqueous solution of ATPES. Then assemble the liquid inlet and outlet pipes, as well as the Ag/AgCl electrode.

Sealing test of individual carbon nanotube nanofluidic chip and the negligible effect of concentration polarization
Concentration polarization is generally not an issue when dealing with individual nanochannel. Despite the large ionic transport that is observed on such devices compared to what expected with standard hydrodynamics, ionic current in the range of pA or nA are not enough to build up a charge polarization region at the extremity of the CNT. Experimentally this is confirmed by the linearity of the current vs voltage ( Figure   S3a) and by the fact the current is stable over time ( Figure S2). This point is also confirmed by the diffusion measurement with reverse high and low concentration for a salinity gradient of 1000 with KCl solution. The absolute value of reverse current is 9.63 pA which is very close to the osmotic measurement current 9.61 pA. Therefore, concentration polarization has no effect on the transport studied in our manuscript.

The calculations of slip length
As presented in the main text, the surface conductivity of the single double-walled CNT can be expressed as

Measurement for DWCNT with an inner radius of 2.7 nm
To verify the reliability of the theoretical model, geometry variation is very useful as suggested by the reviewer. Therefore, we performed an additional experiment with a larger tube. The fabrication method and measurement setup are consistent with the manuscript. The structure of the CNT is characterized using a combining approach of atomic transmission electron microscope (TEM) and Raman spectroscope. TEM image shows the double-walled structure of the tube unambiguously ( Figure S4a), with an outer diameter of 6.1 nm and inner diameter of 5.4 nm, which is a bit larger than that used in the manuscript (inner diameter of 4.6 nm). From Raman data, the disappearance of the D peak and the shape of the G peak ( Figure S4b) show that the CNT is free of defects and belongs to a semiconductor tube. This is the largest DWCNT we could find as the diameter of the DWCNT can only be less than 6 nm (3). The sealing performance tests were performed at first as described in the manuscript, then we explored the ionic transport under a voltage drop. The applied voltage drop is from −1 V to +1 V, and the concentration of the solution is increased sequentially from 10 −3 M to 1 M, as shown in Figure S4 (c-e). Based on these measurements, the slip length of this CNT is estimated to be 17.4 μm, which agrees with the variation of slip length versus the radius of the nanotube ( Figure S4f). Therefore, it is reasonable to believe that the theoretical model considering the interfacial transport of the ions is reliable.

The contribution of mobility difference to the total power
Considering the experimental conditions and using the standard expression for the zero-current potential Em (equation (1) in Esfandiar et al's paper (4)), Em is estimated to be about +0.5 mV for KCl solution with a salinity gradient of 1000. This leads to a current in the order of 0.1 pA (almost 2 orders of magnitude lower than diffuso-osmosis current measured in our experiment). Therefore, the contribution of mobility difference to the total power can be negligible compared to diffuso-osmosis current in our experiment.